Disk drives are digital data storage devices that can store and retrieve large amounts of data in a fast and efficient manner. A typical disk drive includes a plurality of magnetic recording disks that are mounted to a rotatable hub of a spindle motor and rotated at a high speed. An array of read/write heads is disposed adjacent to surfaces of the disks to transfer data between the disks and a host computer. The transducers can be radially positioned over the disks by a rotary actuator and a closed loop digital servo system, and can fly proximate the surfaces of the disks upon air bearings.
A plurality of nominally concentric tracks can be defined on each disk surface. A preamp and driver circuit generates write currents that are used by the transducer to selectively magnetize the tracks during a data write operation and amplifies read signals detected by the transducer from the selective magnetization of the tracks during a data read operation. A read/write channel and interface circuit are connected to the preamp and driver circuit to transfer the data between the disks and the host computer.
A disk drive requires a voltage supply to operate. Typically, the voltage supply is provided by a host computer or storage rack in which the disk drive is mounted. A typical voltage supply provided by a host computer for a disk drive is a DC supply voltage providing +5 volts of DC power and sometimes +12 volts of DC power. In general, the voltage supply to the disk drive is turned on when the host computer is powered up, and turned off when the host computer is powered down. Powering up and down of a host computer is typically a controlled event in which a shutdown request is provided to the CPU of the host computer. The host computer is then allowed to shut down the system in an orderly fashion. For example, buffers may be processed and/or queues may be cleared to ensure that no data is lost during the shutdown. With respect to a disk drive, after all outstanding data reads and writes have been processed, the host computer may turn off the supply voltage to the disk drive.
From time to time, however, the supply voltage to the disk drive may be interrupted unexpectedly. For example, a user may manually unplug or switch off the host computer, or a system controller may cause the power to be turned off and then on. after detecting high power consumption of the disk drive. In some cases, a user may manually turn off and restart a computer if the disk drive fails to operate as expected. Turning a host computer off and back on may cause the power supply to the disk drive to be interrupted and restarted within a short period of time. Such interrupting and restarting of the power supply to the disk drive within a short period of time may be referred to as “cycling” or “recycling” the power supply. For example, if the disk drive fails to spin up to speed when the computer is booted, or the drive may fail to otherwise start properly, the user or system may cycle the power off and back on in an attempt to cause the disk drive to recover. In that case, however, the disk drive may follow the same start-up procedures that failed in the first place, which may lead to repeated failures.
While information about the state of a disk drive may be stored in nonvolatile memories, such memories may be expensive. Moreover, the additional circuitry and programming required to store state information in a nonvolatile memory may increase the complexity and/or cost of the disk drive. Furthermore, an internal fault may have occurred that would make it difficult to write a nonvolatile memory.